The present invention relates generally to solid oxide fuel cells, and more particularly to such fuel cells having thin film electrolytes and thick film electrodes.
There is considerable current research and industrial activity on the development of PEM-based fuel cell systems for Micro-Power applications. The most common PEM systems proposed would use either hydrogen or methanol as a fuel. Hydrogen represents a challenge for fuel handling and distribution. Methanol may be promising in a Direct Methanol PEM fuel cell, but a reduction to commercial practice has not been demonstrated to date. Further, methanol has a relatively low (approximately one half) specific energy as compared to other hydrocarbon fuels such as, for example, butane, propane, gasoline, and diesel. Reported power densities from PEM cells seldom exceed 400 mW/cm2.
Solid Oxide fuel cells (SOFC) have been shown to offer the potential for internal reforming, as well as reported power densities as high as 1900 mW/cm2. A schematic representation of an SOFC is shown in FIG. 1, wherein V0oo stands for oxygen vacancy. The oxygen reduction reaction (taking place at the cathode) is:O2+4e−→2O2−.The O2− ion is transferred from the cathode through the electrolyte to the anode. Some typical fuel oxidation reactions (taking place at the anode) are: 2H2+2O2−→2H2O+4e−  (1)2CO+2O2−→2CO2+4e−  (2)The oxidation reaction at the anode, which liberates electrons, in combination with the reduction reaction at the cathode, which consumes electrons, results in a useful electrical voltage and current through the load.
The application of “thin film” processing techniques has been reported to reduce the practical operating temperature of SOFC from a range of 800° C. to 1100° C., down to about 500° C. or less.
It has also generally been believed that a “thin” electrolyte layer should not be too thin, and thicknesses less than 10 μm have been discouraged in order to avoid the possibility of short circuiting. Some researchers have attempted to provide an improved colloidal deposition technique over the prior technique—prior attempts to use colloidal deposition to deposit films thicker than 10 μm in a single step coating had previously resulted in cracking of the film after drying.
The “thin” film SOFCs are not, however, the SOFCs having the highest demonstrated performance to date. The higher performance/higher power density SOFCs are generally operated at higher temperatures, and use cermets and thick film processes for anode and cathode fabrication. These high performance SOFCs use “thin” film electrolytes; however, these “thin” film electrolytes generally have thicknesses of about 40 μm or more and are fabricated by electrochemical vapor deposition (EVD), tape casting, and other ceramic processing techniques.
A known thin film SOFC 100 is shown in FIG. 2. SOFC 100 comprises a substrate 102 having thereabove a nitride layer 104, a thin film nickel anode 106, a thin film electrolyte 108, and a thin film silver cathode 110.
Some previously known SOFCs have been electrolyte supported (wherein the electrolyte layer provided some structural integrity and was thicker than either the anode or the cathode); cathode supported (wherein the cathode layer provided some structural integrity and was thicker than either the anode or the electrolyte); or anode supported (wherein the anode layer provided some structural integrity and was thicker than either the cathode or the electrolyte).
Fabrication has generally been recognized to be one of the major problems inherent with SOFC. This is due to the fact that all of the components (anode, cathode, electrolyte, interconnect material, etc.) should be compatible with respect to chemical stability and mechanical compliance (eg. thermal expansion coefficients). The layers also should be deposited such that suitable adherence is achieved without degrading the material due to use of too high a sintering temperature. These requirements have heretofore rendered successful and cost effective production of high performance SOFCs very difficult.
Thus, it would be desirable to provide a SOFC and method of fabricating a SOFC which overcome the above-mentioned drawbacks.